The present invention relates generally to medical imaging, and more particularly to opthalmic imaging.
Diagnostics for eye disorders typically include a detailed opthalmic examination of the retina. For initial examination, an eye doctor will view the retina through an opthalmoscope. For a permanent record, the retina is typically photographed with a fundus camera. A fundus photograph directly records various features of the retina, such as blood vessels and lesions. The imaging capabilities of fundus photography may be enhanced by supplementary techniques. A high-contrast image of retinal blood vessels, for example, may be photographed after the injection of a fluorescent dye into the bloodstream. The resulting image is referred to as a fluorescein angiogram (FA).
Fundus photography is not limited solely to images of the retina. The ocular fundus (the inner lining of the eye) comprises the sensory retina, the retinal pigment epithelium, Bruch's membrane, and the choroid. Imaging of the choroid, a layer deeper than the retina, for example, may be produced by injecting indocyanine green dye, which fluoresces in the infrared spectrum, into the bloodstream. The resulting image, which may be captured on infrared film, is referred to as an indocyanine green chorioangiogram (ICG). Fundus images in both the visible and infrared spectrum may also be captured by digital imaging systems instead of photographic film.
More sophisticated techniques have recently been developed for diagnostics of the eye. One such technique is three-dimensional optical coherence tomography (3-D OCT). In this technique, a light beam is directed onto the retina. Part of the beam is back-reflected. Interferometric analysis of the back-reflected light yields information on the structure of the retina. By varying optical parameters of the light probe, features at different depths below the surface of the retina may be probed. With this process, an image of a cross-section of the retina may be generated by scanning the optical probe along a line on the retina. By rastering the optical probe across the surface of the retina, a series of cross-sectional images may be produced. The series of cross-sectional images characterize the 3-D structure of the retina.
Cross-sectional images yield diagnostic information complementary to that provided by fundus images. A more comprehensive analysis of the retina may be provided by analyzing a series of cross-sections of a specific feature, such as a lesion or blood vessel. Accurate alignment of 3-D OCT cross-sections with a feature on a fundus image has proven to be difficult. What is needed, for example, is a high-resolution technique for spatially mapping 3-D OCT measurements with features on a fundus image.